The present invention relates to the nuclear medicine art. It finds particular application in conjunction with two head single photon emission computed tomography (SPECT) camera systems and will be described with particular reference thereto.
Early nuclear or Anger cameras had a single radiation detector head which was positioned stationarily over a region of interest of the subject. The subject was injected with a radioactive dye which circulated through the patient's circulatory system. Some of the radiation given off by the dye was received by the nuclear camera head which converted the radiation event into light.
More specifically, the nuclear camera head included a scintillation plate which converted each radiation event into a scintillation or flash of light. An array of photomultiplier tubes positioned in back of the scintillator plate and associated circuitry determined an (x,y) coordinate location and an energy or (z) value for each scintillation event. A collimator including a grid-like array of lead vanes limited the path or trajectory of radiation events which could strike the scintillation plate. Typically, the collimator constrained each incremental element of the scintillator plate to be receptive only to radiation directly in front of it, i.e., radiation along paths substantially perpendicular to the scintillator plate. In this manner, a shadowgraphic image of the frequency of radiation events in the examined region of the subject was developed.
When the detector head was rotated around the subject or indexed to a multiplicity of angularly offset positions around the subject, a data set was collected which is the mathematical equivalent of a CT scanner data set. More accurately, because the nuclear camera head is two-dimensional, a series of data sets were collected which each corresponded to one slice of an imaged volume.
As with CT scanners, rotating the detector head 180.degree. around the subject produced a complete data set. For faster or more detailed imaging, two detector heads have been mounted to the gantry for rotation around the patient. Typically, the two detector heads have been placed 180.degree. opposite to each other. The 180.degree. opposite orientation has numerous advantages. Mechanically, the detector heads and their lead collimators are very massive. By positioning a pair of detector heads opposite to each other, a counterbalance effect is achieved which simplifies or reduces the strength of mechanical bearings, gears, and actuators needed for rotating the gantry. A second advantage resides in the reconstruction process. When the heads are positioned 180.degree. opposite to each other, both heads view the same rays or paths through the subject. This doubles the data acquisition rate and enables data collected by the two heads to be combined in real time for computational efficiency. Of course, when the heads are positioned 180.degree. opposite to each other, 180.degree. of rotation is still required to generate a complete data set.
Rather than placing the two detector heads 180.degree. apart on the gantry, advantages have been achieved by placing the detector heads 90.degree. apart. When two detector heads are placed 90.degree. apart, a complete data set can be collected in only 90.degree. of rotation. Note that during the first 90.degree. of rotation, each detector head is viewing unique rays. After 90.degree. of rotation, each detector head starts receiving data along rays which the other detector head previously sampled.
Regardless whether the detector heads are placed 180.degree. apart or 90.degree. apart, it is advantageous to position the patient as close as possible to the detector head. Because the human torso is generally not circular, the detector heads are typically movable radially from an axis of rotation such that they can follow the contours of the patient's body.
Heretofore, gantries with 180.degree. opposed detector heads have been adapted to reposition the detector heads 90.degree. apart. However, repositioning the detector heads 90.degree. apart has been a relatively complex mechanical movement. After being positioned in the 90.degree. apart position, the heads are not movable radially about the center of rotation. In one system, the heads are so large that they touch at their corners before reaching a minimal spacing from the patient. In another system, the mechanical arrangement which enables the heads to be shifted between 180.degree. and 90.degree. cannot accommodate radial movement. To maintain the minimal patient/detector head spacing in the 90.degree. detector position, the prior art systems move the patient relative to the detector heads. That is, while the detector heads rotate a fixed radius from the center of rotation, the position of the patient is shifted along vertical and horizontal axes to maintain the patient a minimal distance from the detector heads.
The present invention provides a new and improved SPECT camera system which provides the benefits of both 180.degree. opposite and 90.degree. position detector heads.